The present invention is directed to sound sensors for detecting underwater sound. It is specifically directed to optical methods of sound detection.
There are numerous applications for underwater sound detection in exploration, navigation, and warfare. Recent attempts have been made to use optical means for detecting the sound. For instance, in U.S. Pat. No. 3,474,253 to Kessler, a device is described in which coherent light is sent through water in the vicinity where sound is to be detected, and the frequency shift due to Brillouin scattering is detected to determine the presence of sound in the region between a light detector and the source of the coherent light. Whatever the virtues of this method may be, it requires that both the light source and the light sensor be positioned at the location where the sound is to be sensed. Accordingly, if the Kessler device is to be used for remote detection of sound, a rather elaborate and expensive device carrying a source of power must be left at the position to be monitored.
An alternate arrangement that solves some of these problems is described in U.S. Pat. No. 3,611,277 to Yoder. In the Yoder system, a source of coherent light may be positioned on, for instance, a helicopter. The coherent light is directed to a floating reflective device that sits in the water and includes a reflective diaphragm. The membrane contacts the water on one side so as to vibrate in response to sound in the water. The float contains a beam splitter for splitting the received coherent light into two beams, one beam being directed at the reflective diaphragm and the other beam being directed at a stationary reflector also contained in the float. Light from both the stationary reflector and the diaphragm is reflected back to a sensor in the helicopter, where the resultant amplitude variations caused by the changing phase relationship between the interfering beams are detected to provide an indication of the presence of sound in the water.
With the Yoder arrangement, the occurrence of sound at a remote location is detected without the necessity for any mechanical connection between the detection electronics and the float at the remote location. Furthermore, the device left at the remote location is a passive device and so does not need a power source that eventually requires replacement. However, the Yoder device, while considerably simpler than that described in the Kessler patent, nonetheless has some mechanical complexity. In particular, the reflective diaphragm must be mounted in such a way as to permit motion with respect to its mounting structure and to provide an interface between seawater and the interior of the device. Furthermore, the spring-mass system of the diaphragm and its mounting necessarily have an effect on the detected signal, and care must be made in design to minimize this effect.
It is accordingly an object of the present invention to detect underwater sound optically by employing a method in which the device that is to be left at a remote location is simple and inexpensive and responds faithfully to the sound signal.